Variably damped passive tank stabilizer



Jan. 21, 1969 s. B. FIELD ETAL 3,422,781

VARIABLY DAMPED PASSIVE TANK STABILIZER Filed May 8,1967 Sheet of 4 GYROSCOPIC CONTROL UNIT INVENTORS. SHELDON B. FIELD, FRANS V. A, PANGALILA ORNEYS.

Sheet S. B. FIELD ET AL VARIABLY DAMPED PASSIVE TANK STABILIZER WHHI H HHU Jan. 21, 1969 Filed May 8, 1967 I3LI-- &1

INVENTORS SHELDON B. FIELD, FRANS v. A. PANGALILA BY Q I my ORNEY-SI Jan. 21, 1969 Filed May 8, 1967 AMPLITUDE OEROLL IN DEGREES s. B. HELD ET AL 3,422,781

VARIABLY DAMPED PASSIVE TANK STABILIZER Sheet of 4 CURVE FOR UNSTABILIZED SHIP RESONANCE FRAHM TYPE STABILIZER IDEAL CURV FREQUENCY OF ROLL, H

c c c CURVE USING STABILIZER OF U.S. PATENT N0. 3,054,373

-XX- B=CURVE USING STABILIZER OF INVENTION INVENTORS. SHELDON B. FIELD, FRANS V. A. PANGALILA TTORNEYI Jan. 21, 1969 Filed May 8, 1967 S. B. FIELD ET AL VARIABLY DAMPED PASSIVE TANK STABILIZER Sheet 4 014 6 SYSTEM POwER SUPPLY 3| ROLL RATE SENSING COMPUTER a S TRANSDUCER (GYRO) SETTING CONTROL 76 AND OVERRIDE PANEL 5; UNSTABILIZED SHIP CD L|..| O

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FREQUENCY OF ROLL]? INVENTORS SHELDON B. FIELD, FRANS v. APANGALILA BY mmwgm ATTORNEYS United States Patent 3,422,781 VARIABLY DAMPED PASSIVE TANK STABILIZER Sheldon B. Field, Floral Park, and Frans V. A. Pangalila, Staten Island, N.Y., assignors to Flume Stabilization Systems, Inc., Hoboken, N.J., a corporation of New Jersey Filed May 8, 1967, Ser. No. 636,780 US. Cl. 114125 Int. Cl. B63b 43/06 ABSTRACT OF THE DISCLOSURE A passive tank stabilizer for ships including an elongated tank partially filled with a body of liquid in a free surface condition, and a gyroscopically controlled door operative between open, closed and intermediate positions so as to variably and selectively control the amount of damping imparted to transferring liquid, said door operatively permitting maximum liquid passage when the vessel rolls near natural frequency and closing by degrees as the ship rolls at frequencies further away from resonancy and being closed to deactivate the tank when the ship is forced rolled above at frequencies at which the stabilizer would otherwise destabilize the ship.

Claims Background The present invention relates to passive tank stabilizers and more particularly to substabilizers of the free surface type in which by virtue of the tank geometry and the liquid level of the stabilizin medium therein the tank liquid oscillates with a natural frequency matched to the roll of the ship.

At the turn of the twentieth century, important advancements were made in the art of roll stabilizing ships at sea primarily by the contributions of Herman H. Frahm. Frahms stabilizer, commonly known as a U-tube type passive tank, was characterized by a pair of opposite wing tanks interconnected by a narrow duct which extended from the bottom of the wing tanks down around the bottom of the ship to the bottom of the opposite tank. In later designs, the entire tank was confined between decks of a ship but the interconnecting duct continued to have a height to confine the liquid in the crossover duct. With this arrangement, the Frahm tanks oscillated at a frequency tuned to the roll of the ship so that the stabilizing moment was applied first in one direction and then in another depending upon the angular position of the ship. In view of Frahms basic assumption that a ship rolls at its own frequency regardless of sea, the Frahm approach operated generally satisfactory when the ship rolled at its natural or designed frequency; however, the primary reason that the Frahm tanks fell into disuse was that when the ship was forced rolled at a frequency somewhat removed from the natural roll frequency, the tanks increased or introduced rolling and thereby created an unsafe condition at sea. Corrective changes were made in the design by incorporating a manually or automatically controlled valve in a crossover air duct communicating with the upper parts of the wing tanks. An operator, upon sensing a variation in the roll frequency, or upon visually detecting a change in the oncoming wave frequency, would operate the valve to throttle the air passage and in this way attempt to dampen liquid transfer between the wing tanks. This technique not only proved unsatisfactory from the stabilization aspect, but it was also unbearably noisy in operation.

Following the Frahm period, the art developed away from tank stabilizers and turned to the use of activated "ice fins such as the type installed on the Queen Mary which although extremely expensive to build and install operated satisfactorily when the ship was in motion. Because of the excessive installation and maintenance expense and the undesirable effect on speed, fins have enjoyed only limited use.

With the advent of missile tracking ships in the late fifties, there developed a need for a stabilizer which would operate when the ship is substantially at rest. To meet this need, a new passive tank stabilizer was developed and patented in the United States No. 3,054,373. Unlike the Frahm tanks, this later development relies only on hydrodynamic damping to control the passage of liquid from one end of the tank to the other. The tank is characterized by having substantially the same height throughout with a liquid body partially filling the tank so that an air space is formed above the liquid thus giving the liquid a free surface condition throughout the tank. Tuning control is accomplished by adjusting liquid height. Unlike the Frahm tanks, the response curve for the stabilizer is substantially flat. When designing the tank parameters, some sacrifice is made in performance near resonance to achieve suitable results when the ship is force rolled above or below resonance. Thus, although a ship stabilized by the free surface tank has a slightly greater roll amplitude at resonance than the same ship stabilized with a Frahm type stabilizer, the conventional free surface tank does not destabilize the vessel nearly as much as the Frahm tank at high and low frequencies removed from the resonant frequency.

It is generally known that the free surface tank is currently setting the standards for the industry and tanks of this type have been installed in over three hundred vessels since 1960. However, a free surface tank with its fixed geometry suffers from the disadvantage that although its performance characteristics are much better than the Frahm type system at high and low frequencies, the conventional tank still introduces slightly more roll amplitude to the ship at these frequencies than is the case with the tank deactivated. In addition, because the tank must be designed to introduced substantial amounts of damping to the tank liquid for suppression of destabilization at the roll frequency extremities (particularly the low frequency), the stabilization moment magnitude at resonant frequency is unduly suppressed.

The present invention improves on the conventional free surface tank by providing for the first time variable damped, free surface tank which senses the roll frequency and amplitude of the ship and automatically controls damping so that damping is reduced when the ship rolls near resonance and the damping is maximized to completely deactivate the tank when the ship rolls beyond a frequency limit. The damping may be variably set to intermediate degrees at specified frequency ranges between the frequency limit and resonance.

To accomplish these results, the present invention provides a passive free surface tank with a motor control valve or door movable between fully opened and fully closed positions. An automatic sensing and control unit, preferably a gyroscopic type, senses the rolling of the ship and produces signals of predetermined magnitude corresponding to the rolling frequency and, in some cases, amplitude. When the Ship rolls near resonance (which for the purpose of this disclosure is synonymous with natural roll frequency) the unit controls the motor to open the door as wide as possible so as to impart minimum damping and to permit maximum liquid passage therethrough resulting in the greatest stabilizing moment. When the ship rolls at a frequency below a predetermined minimum frequency and, in some cases, above a predetermined maximum frequency, the unit controls the motor to fully close the door thus deactivating the tank preventing the tank from destablizing the ship. At this time, the ship is stablized by virtue of its inherent static stability. At intermediate ranges of frequencies, the control unit operatively moves the door to intermediate positions so as to impart controlled and predetermined amounts of damping depending upon the sensed rolling frequency.

It is, therefore, a primary object of the present invention to provide a variable damped free surface tank stabilizer which provides the advantages and solves the problems outlined above.

Description of drawings Other and further objects of the invention will become apparent with the following detailed description when taken in view of the appended drawings in which:

FIGURE 1 is a horizontal section of a tank in accordance with the present invention and part of the ships hull.

FIGURE 2 is a transverse vertical section taken along line 22 of FIGURE 1.

FIGURE 3 is a longitudinal vertical section taken along line 33 of FIGURE 1.

FIGURES 4 and 5 are charts illustrating the amplitudefrequency characteristics of an unsta blized ship, and ships stablized with various stablizer configurations including the present invention.

FIGURE 6 is a typical block diagram schematic of one example of the sensing and control apparatus for the invention.

Detailed description With reference to FIGURES 1-3, a preferred embodiment of the invention includes a passive tank stablizer generally indicated as 10 arranged between adjacent decks 12 and 14 of ship 16 and extending from one side of the hull to the other. The hight of tank 10 is generally uniform throughout and a body of liquid 18 partially fills tank 12 to a level adjusted so that the natural frequency of the tank liquid oscillation is matched to the roll frequency of the ship. The breadth (fore and aft) of tank 10 is determined by the particular volume capacity and geometry selected to best suit the particular ship being stabilized. The liquid can be of any suitable kind such as fresh 'water, sea water, bunker oil, fuel or the like.

According to the principles of US. Patent No. 3,054,- 373, tank 10 is divided into two wing tanks 11 and an interconnecting channel 13 by four upstanding members 32 aligned in pairs and spaced from the center and ends of the tank. Members 32 extend from the bottom of the tank up to a height well above the static surface of body 18 and have mutually facing ends 34 spaced from each other to form passageways 36 and turned outward for hydrodynamic purposes. The cross section area of opening 36 is designed so that members 32 impart some damping to the passage of liquid therethrough. For reasons made clear below, the damping provided by members 32 should be less than that provided by the tanks of U8. Patent No. 3,054,373.

According to the invention, a damping and cut-off control assembly 20 is arranged in the center of the tank. The assembly can have any suitable form such as sliding doors, rotating horizontal or vertical flaps, or the like. One preferred assembly is formed by a pair of upstanding plates 22 extending from the floor of tank 10 up the fore and aft walls thereof to a height suitably above the expected static liquid level within the tank. A valve plate or door 24 supported for rotation about a vertical shaft 26 between plates 22 is operatively movable between perpendicular and generally parallel positions relative to plates 22, i.e., open and closed positions, respectively. The distance D should be selected so that door 24 when in a partially opened position cooperates with members 32 to impart damping to liquid passage.

Door 24 is operated by a reversible motor 28 receiving control signals from a gyroscopic control unit 3 1 which senses the frequency of the rolling of the ship and generates a control signal which operates motor 28 to effect rotation of door 24 to the fully opened, fully closed, or one of a number of intermediate positions. In this way, the assembly 20 operates to vary the damping imparted to the tank liquid and in some instances serves to de-activate the tank completely. It will be appreciated that more than one such assembly 20 and control motor therefor can be provided throughout the tank as desired. Mechanical means (not shown) may be provided to bias the door toward the closed position so that the tank halves are isolated in the event of power failure. Motor 28 is housed within a space formed in the top of the tank by a flat plate 30 supported by the tops of plates 22 and 32 and joined rigid with the top of the tank by transverse diaphrams 38 and longitudinal plates 40, the latter of which are provided with openings 42 to permit a free passage of air and to reduce the weight of the assembly.

In order to achieve smooth and reliable operation for the damping and cut-off assembly 20, it is preferred that unit 31 average the frequency of the previous ten (or other suitable number) roll cycles and generate control signals accordingly. 'In this way, the stabilizer will avoid hunting and will not be adversely affected by transients as would be the case if assembly 20 responded solely to the frequency of each roll cycle.

Unit 31 may comprise any suitable mechanical or electrical arrangement, one example of which is illustrated in FIGURE 6. Power supply 71 applies operating power to the system. A sensing transducer stage 72 senses the roll rate and magnitude and develops a sinusoidal signal accordingly. The transducer feeds this signal to the computer stage 74 which is programmed by the manually adjusted switches of the control panel 76 which has override switching capability. The computer automatically develops the door control signals in accordance with the control switch settings and receives transducer signals and applies the same to motor 28. In response thereto, motor 28 opens, closes, or moves door 24 to one of a number of intermediate positions. Besides the averaging technique mentioned above, computer 7-4 can also generate a hold signal to fix the door 24 in position whenever the sensed roll magnitude is below a predetermined value regardless of roll frequency.

The operation of the invention is best understood with reference to FIGURE 4. In that figure there is illustrated the typical amplitude-frequency characteristic curve for the unstabilized ship. Roll amplitude is maximum at resonance, F that is, when the frequency of oncoming waves is matched to the natural roll frequency of the ship. It can be seen that because of the static stability of the ship the amplitude of roll at frequencies well above and below F drops off to only a fraction of that at resonance. Also illustrated in the figure is the curve for the typical Frahm type stabilizer with its characteristic double hump bridging the resonant frequency. It is thus clearly seen that the Frahm type stabilizer introduces more roll amplitude at the high and low frequency regions than for the case of the unstabilized ship, However, the Frahm type stabilizer does achieve good results near resonance.

Also shown in FIGURE 4 is curve A for a typical passive free surface tank of the type disclosed in US. Patent No. 3,054,373 which provides a generally flat response curve A showing adequate roll reduction near resonance and much better results at :high and low frequencies than in the case of the Frahm type stabilizer.

The amplitude-frequency roll characteristics of a ship stabilized :by the present invention is represented by curve B. When the ship rolls at a frequency near resonance (F control unit 31 operates motor 20 to open door 24 to the fullest extent. With door 24 fully opened, the tank imparts a minimum damping to the liquid passage so that a maximum stabilizing moment is developed in op position to roll. Because of the operation of the invention at high and low frequencies as described below, the magnitude of this minimum damping (developed by all internal members 32, 22, 24) is less than that developed by the conventional free surface tank of US. Patent No. 3,054,373. Therefore, for frequencies near F curve B has a lower magnitude than curve A. Like the tank in said patent, the invention is designed so that the tank liquid oscillates at a substantial phase lag relative to the ships roll, i.e., up to a theoretical 90 lag.

When the gyroscopic control unit 31 senses that the ship is rolling below F or above F it generates a signal sufficient to close door 24 an amount which corresponds to the extent said frequency is removed from F or F Thus, as the ship rolls at decreasing frequencies below and relative to F or at increasing frequencies above and relative to F door 24 is rotated to more closed positions to correspondingly block liquid passage. As evident from FIGURE 4, the effect of closing door 24 in the regions of F -F and i -F is to change the response curve toward that for the unstablized ship at the frequency limits. Examples of the door position-roll frequency are:

Roll frequency: Door position:

F and below Full closed. Near F /3 open. Near F open. F and above Full open. F and below Full open. Near F 73 open. Near F /3 open. F and above Full closed.

If the control senses the ship rolling at or below F or at or above F door 24 is fully closed to thus deactivate the stabilizer so that the ship rolls with an unstabilized characteristic which is an improvement over conventional free surface tanks and, of course, over the Frahm type stabilizer. When door 24 is completely closed to deactivate the tank, plates 32 assist to quiet the liquid on each respective side of door 24.

Another mode of operation for the invention involves stabilizing short period ships. For these ships a greater volume of tank liquid may be placed in the tank than is required for tuning the tank liquid and ship roll. Normally, such a step would be detrimental to tank operation at low frequencies but beneficial to tank performance near F and high frequencies. See curve C, FIGURE 5. But with the invention good results are obtained over the entire frequency range by effecting curve D at frequencies below F To develop curve D, the invention operates as follows:

Roll frequency: Door position F and below Full close. Near F /1 open. Near F /2 open. Near F /1 open. F and above Full open.

Thus, there has been described a new and improved variable damped free surface passive tank stabilizer which is designed for optimum performance near resonant frequency without consequence to the destabilization effect it might have at high or low frequency forced roll conditions. In addition, the invention provides means to impart variable amounts of damping to the tank liquid at prescribed frequency ranges so that the ship operates with improved stability.

What is claimed is:

1. A roll stabilization system for ships comprising an elongated tank extending from one side of the hull to the other, a body of free surface liquid distributed generally throughout the tank and partially filling the tank to a level at which the tank liquid imparts to the ship an oscillating, timed stabilizing moment near the natural frequency of the ship at a phase lag significantly near closure means including a closure assembly located within the tank and spaced from the ends thereof to selectively isolate and permit liquid communication along the liquid path within the tank, and sensing and control means for sensing the roll frequency of the ship and closing the closure assembly to effectively deactivate the tank when the average of a predetermined number of roll cycles is at a first frequency away from its natural frequency and at which the tank liquid would destabilize the ship so that the ship rolling characteristic is primarily dependent upon the ships static stability.

2. A system as set forth in claim 1 wherein the sensing and control means elfects a first opening of said closure means when the ship rolls at a second frequency between the ships natural roll frequency and said first frequency in order to permit liquid passage in the tank with a first degree of hydraulic damping.

3. A system as set forth in claim 2 wherein the sensing and control unit effects a second opening of the closure assembly greater than the first opening when the ship rolls at a third frequency between the ships natural roll frequency and said second frequency in order to permit liquid passage in the tank with a second degree of hydraulic damping less than said first degree.

4. A system as set forth in claim 2 wherein the tank imparts a fixed hydraulic damping to the tank liquid so that variation of the closure assembly varies the total hydraulic damping in the tank.

5. A system as set forth in claim 4 wherein said closure assembly is fully opened when in the second opening position so that the hydraulic damping in the tank is a minimum for a given liquid level.

6. A system as set forth in claim 5 wherein the tank geometry is such that when the closure assembly is fully open the tank liquid is underdamped for maximum volume passage per roll cycle.

7. A system as set forth in claim 3 wherein the liquid level is set so that the natural tank liquid oscillation is tuned to the ships roll with the closure assembly full open and minimum damping being imparted to liquid passage.

8. A system as set forth in claim 7 wherein said first and second frequencies are below the ships natural roll frequency.

9. A system as set forth in claim 7 wherein said first and second frequencies are above the ships natural roll frequency.

10. A system as set forth in claim 3 wherein the liquid level is selected above that required for tuning the tank liquid oscillation frequency with the ships roll and said first and second frequency are below the ships natural roll frequency.

References Cited UNITED STATES PATENTS 3,192,888 7/1965 Field 114-125 3,269,346 8/1966 Bell 114125 TRYGVE M. BLlX, Primary Examiner. 

